UNIFORM SILICON ISOTOPE RATIOS ACROSS THE MILKY WAY GALAXY AND IMPLICATIONS FOR ISO-TOPIC GALACTIC CHEMICAL EVOLUTION. Nathaniel N. Monson1, Mark R. Morris2, and Edward D. Young1,1Department of Earth, Planetary, and Space Sciences, UCLA, USA (nn.monson@ucla.edu, eyoung@epss.ucla.edu),2Department of Physics & Astronomy, UCLA, USA (morris@astro.ucla.edu).

Introduction: Galactic chemical evolution (GCE) oflight stable isotopes is central to the interpretation of theorigins of presolar grain isotope ratios and indeed thoseof the solar system itself. GCE leads to shifts in isotoperatios over time in what should be broadly predictableways. The shifts are especially pronounced for ratios ofsecondary nuclides to primary nuclides, and the detailsof the process are clearer where two or more such ratiosare available. When studied as functions of distance fromthe Galactic center, RGC, isotopic abundance ratios de-lineate the extent of stellar processing within the Galaxy,and serve as signposts for chemical variations with time[1, 2, 3]. The secondary/primary isotopic abundance ra-tios of oxygen [e.g., 4, 5] and carbon [e.g., 6, 7, 8] havehave been used extensively to trace variations in the de-gree of astration across the Galaxy.

The existence of two secondary isotopes makes theoxygen system particularly attractive for tracing GCE.When combined with CO carbon isotopologue data,the CO oxygen isotope data suggest that 18O/16O and17O/16O ratios increase linearly with decreasing RGC

along a slope-1 line in oxygen three-isotope space (plotsof δ′17O vs. δ′18O where δ′iO = 103 ln(Ri/Ri,ref)),in qualitative agreement with the predictions of sec-ondary/primary increases with GCE. However, the rangein oxygen isotope ratios is greater than a factor of 10,exceeding theoretical predictions [4] by a factor of ∼ 2to 3 [9]. Optical depth effects have hampered efforts todetermine C16O column densities within sources, raisingthe possibility that the oxygen CO isotopologue data arenot providing an accurate picture of isotopic GCE.

Testing GCE using SiO: The silicon isotope systemis analogous to that of oxygen in that it contains one pri-mary and two secondary nuclides. The abundant primarysilicon isotope, 28Si, is an alpha process nuclide. 29Siand 30Si are both secondary, forming largely from 25Mgand 26Mg during Ne-burning, as well as during core-collapse Type II supernovae. Both rarer isotopes alsoform from 28Si in the He-burning shells of AGB stars.While contributions from He-burning AGB stars couldalter local compositions, it likely has little effect on theoverall isotopic budget of the interstellar medium (ISM).GCE models predict that, to first order, the silicon andoxygen isotope ratios should evolve in parallel. There-fore, based on the oxygen data, one expects nearly con-stant 29Si/30Si across the Galaxy, as well as radial gra-dients in the 29Si/28Si and 30Si/28Si ratios that increasewith decreasing RGC.

Methods: We report the relative abundances of thethree silicon isotopologues of SiO, 28SiO, 29SiO and30SiO across the Galaxy using the v = 0, J = 1 → 0rotational transitions measured with the Q-band receiveron the Green Bank Telescope (GBT). The chosen sourcesrepresent a range in Galactocentric radii (RGC) from 0to 9.8 kpc. The data have been processed with a newdata reduction scheme including a full assessment of thelikely errors associated with these data.

Optical depth effects plague radio telescope isotopo-logue ratio measurements in general [10]. We thereforedeveloped a method for estimating optical depth for theabundant SiO isotopologue in this study based on com-parisons of line shapes afforded by the high spectral res-olution of the GBT. The underlying principle is that highoptical depths manifest as broadening in the 28SiO linerelative to the rarer isotopologue lines that is obviouswhen the emission lines for the different isotopologuesare scaled by area; the degree of coincidence of lineshapes when scaled by the ratio A28SiO/Ai, where Ai

are the areas of the individual isotopologues, is diagnos-tic of optical depth. With this method, optical depthsin the 28SiO emission lines are determined by analyzingdifferences between the 28SiO and 29SiO and/or 30SiOlineshapes for the same source under the assumption thatthe latter are effectively optically thin. We use a MonteCarlo error analysis in order to assess the uncertaintiesimparted by the optical depth corrections.

Results: Our uncorrected data exhibit a spread upand down the slope-1 line in Si three-isotope space, an-chored by the two Galactic center sources and crudelyresembling the predictions from GCE (Figure 1).

However, correcting for optical depth removes thespread in data, resulting instead in a clustering of the dataspanning the range defined by the mainstream SiC preso-lar grain trend (Figure 2). We find, not surprisingly, thatoptical depths on the order of unity can strongly bias ex-tracted isotope ratios. These results indicate that uncor-rected effects of opacities were responsible for the priorevidence for high 29SiO/28SiO and 30SiO/28SiO ratiosin the present-day ISM relative to solar and meteoriticalvalues [10].

Implications: Our finding that secondary/primarySi isotope ratios have no detectable variation across theGalaxy within about 20% (Figure 3) does not com-port with expectations from the large variation in sec-ondary/primary O isotope ratios of >∼ 900%. Evenwhen accounting for the prediction that the growth of

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Figure 1: Uncorrected SiO silicon isotopologue abun-dance ratios for the seven sources observed as part of thissurvey. Mainstream presolar SiC grain data are shownfor reference (grey circles). The solid line is the slope-unity line through the solar composition. The white cir-cle with dot indicates present-day solar abundances anddefines the origin. Error ellipses are 1σ.

secondary/primary ratios for Si isotopes should be ap-proximately 1/3 that for O over the same range in metal-licity, the observed variation is surprisingly small. Thehigher 29Si/28Si and 30S/28Si ratios of the ISM relativeto solar values presumably represents growth of Galacticsecondary isotopes over the last 4.6 Gyrs. This result isin apparent conflict with the hypothesis that solar Si issubstantially and anomalously enriched in 28Si relativeto the ISM at the time of the birth of the solar system.In light of these conclusions, a careful reexamination ofthe Galactic distribution of oxygen isotopes seems wellwarranted.

The spread in Si isotope ratios found among main-stream SiC grains is similar to the spread in values seenin the modern Galaxy, suggesting that the presolar SiCgrains may record both temporal and spatial evolutionof silicon isotope abundances in the pre-solar Galaxy.The key to the conundrum of the higher 29S/28Si and30S/28Si ratios of some mainstream SiC grains relativeto solar may lie with the spread in grain data rather thanwith the solar value.

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Figure 2: SiO silicon isotope ratios after correcting foroptical depth effects. Error ellipses are 1σ determined byMonte Carlo simulations including the uncertainty in theoptical depth corrections.

Figure 3: 29SiO/28SiO in permil vs. Galactocentric dis-tance for both corrected and uncorrected data. Linearregressions for the uncorrected data (grey) and correcteddata (black) are shown with 95% confidence bands.

3231 (2011). [4] N. Prantzos, O. Aubert, J. Audouze, AAP309, 760 (1996). [5] R. W. Wilson, W. D. Langer, P. F.Goldsmith, ApJl 243, L47 (1981). [6] W. D. Langer, A. A.Penzias, ApJ 357, 477 (1990). [7] W. D. Langer, A. A.Penzias, ApJ 408, 539 (1993). [8] S. N. Milam, C. Savage,M. A. Brewster, L. M. Ziurys, S. Wyckoff, ApJ 634, 1126(2005). [9] E. D. Young, M. Gounelle, R. L. Smith, M. R.Morris, K. M. Pontoppidan, ApJ 729, 43 (2011).[10] A. A. Penzias, ApJ 249, 513 (1981).

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